The invention refers to a hull for a single-screw vessel, a twin-screw vessel with two aftbodies and a catamaran for all Froude numbers and block coefficients, respectively, as usual in merchant shipping.
In conventional hulls for single-screw vessels with a symmetric aftbody form at portside and starboardside it is not possible to influence the water line entry angles differently at the two ship's sides.
As the flow to the propeller is appreciably worse on that side of the ship on which the propeller blades move upwards than on the other side, it is possible to improve the flow to the propeller and the propulsion efficiency by an asymmetric aftbody design. Such an asymmetric aftbody form is described in the Patent Specification DE-PS No. 12 07 820. According to this patent a hull for a single-screw vessel or a vessel with a central screw with a low Froude number and a correspondingly high block coefficient is provided with an asymmetric aftbody shape in such a way that the part above the propeller shaft is twisted in relation to the part below the propeller shaft opposite to the direction of propeller rotation. Thus in way of the propeller post the centre line of the horizontal sections through the hull are inclined away from the longitudinal median plane contrary to the direction of the propeller rotation--this inclination increasing from the roots of the propeller blades to the ends of the blades--so that when seen from aft the propeller post above the propeller shaft forms an angle of inclination with the longitudinal median plane in the opposite direction to that of propeller rotation.
This known hull form was primarily developed for vessels with high block coefficients to improve the flow to the propeller and to increase the aftbody block coefficient as compared with symmetric aftbodies without a reduction of the propeller efficiency. Or, if the block coefficient or the displacement of the aftbody, respectively, remains unchanged, either an increase of speed, or at the same speed, a reduction of propulsion power and thus of fuel consumption could be achieved.
Model tests with various vessels with this known aftbody form resulted in savings of propulsion power or fuel between 5 and 7%. At these vessels the part above the propeller shaft of the aftbody was twisted in relation to the part below the propeller shaft opposite to the direction of propeller rotation, and a conventional U-shaped frame character below the propeller shaft resulted in lines of the isotachs of wake which in this area came very close to the ideal of rotational symmetry. This refers to vessels with block coefficients between abt. 0.75 and 0.83. With respect to faster vessels lying in the range of higher Froude numers and lower block coefficients, it is known that the lines of isotachs can be influenced more favourably by arranging the known stern bulbs. With such stern bulb forms or U-shaped frames below the propellers shaft it is, however, not possible to impart an additional rotation to the water flowing towards the propeller.
It is also known that compared with the symmetric aft body form the asymmetric form exerts a favourable influence on the course behavior and the resistance of the vessel due to the fact that the pressure point of the propeller is shifted towards the centre-line, so that the angle of attack of the rudder for the straight-away course is negligibly small.
For economic reasons it is often advisible to use propellers with maximum diameters and a correspondingly low number of revolutions to attain an improvement in efficiency and a reduction of power or fuel consumption, respectively. Propellers with extremely large diameters in relation to the draft and an insufficient immersion of the blades (in the 12 o'clock position), however, imply the danger that air is sucked in by the propeller resulting in deteriorated wake conditions, vibration-generating forces and cavitation, etc.
To avoid these adverse effects tunnelled aftbodies, guide plates or propeller nozzles were successfully arranged depending upon the particular type of vessel, her block coefficient and speed. Also in case of normally immersed propellers and adequate speeds nozzles are applied which surround the propeller or are installed forward of the propeller on the hull.
For certain ship types, for instance for single-screw Ro-Ro vessels, the arrangement of the engine plant and propeller shaft on one side of the ship can bring about significant technical advantages, as for instance the arrangement of the loading ramp beside the engine room and the like, and this may result in different grades of fineness or block coefficients, respectively, on either side of the aftbody.